The present invention relates to separators and, in particular, it concerns centrifugal separators suitable for separating dry components.
A wide range of techniques are in common use for separating mixtures into their different components. The most common technique used for dry separation involves the use of a vibrating mesh or sieve. However, sieve-based techniques suffer from a number of limitations. When used for very small or irregular particles, the sieve tends to become blocked. Sieves are also unable to separate particles of similar dimensions with different shapes or roughnesses (morphology).
An alternative technique for dry separation employs vibrating surfaces without the use of a sieve. This technique is very slow, and also generates a lot of dust.
A further technique for dry separation involves the use of a stream of air passing through the mixture, tending to carry with it smaller lighter particles. Air-flow based techniques are suitable for separating small particles. However, since they require a two stage process of separation followed by removal of particles from the air stream, they are costly to implement. Air-flow based techniques are also unable to separate particles of similar dimensions and weights which have different morphology.
Because of the limitations and disadvantages of the aforementioned dry separation methods, a range of liquid based, or "wet" separation techniques are in common use. These techniques, based on the different densities of particles, include floatation and centrifugal techniques. However, wet techniques have additional disadvantages. Firstly, mixing with water may damage certain materials. Even where damage is not caused, considerable additional time and expense is involved in subsequent drying of the materials. Here too, particles cannot be separated according to their morphology.
Turning now specifically to the prior art centrifugal techniques, examples are disclosed in U.S. Pat. No. 489,101 to Seymour; U.S. Pat. No. 736,976 to Keiper; U.S. Pat. No. 1,712,184 to Wendel; U.S. Pat. No. 1,750,860 to Rawlings; U.S. Pat. No. 1,935,547 to Dryhurst; U.S. Pat. No. 2,415,210 to Hoefling; U.S. Pat. No. 3,366,318 to Steimel; U.S. Pat. No. 4,072,266 to Dietzel; U.S. Pat. No. 4,608,040 to Knelson; U.S. Pat. No. 5,300,013 to Frassdorf et al.; and U.S. Pat. No. 5,372,571 to Knelson et al. These examples typically employ a revolving bowl with a liquid working medium in which heavy particles settle downwards while lighter particles are carried upwards by the liquid flow. The physical process involved is an interaction between gravitational settling and liquid flow forces.
Worthy of particular mention for its resemblance to the present invention, U.S. Pat. No. 1,935,547 to Dryhurst discloses a rotating bowl with riffled inserts for separating ore. Although use of a liquid working medium is not mentioned explicitly, the somewhat terse description describes the coarse riffles as retaining the heavier particles while the while the dirt and other refuse passes over (see lines 78-85). However, it is clear that, in the absence of a liquid working medium, the opposite would occur, with small particles becoming trapped within the riffles. Furthermore, the near vertical walls of the bowl and the additional barrier formed by V-shaped corners of the riffles would render the device inoperative in the absence of a liquid medium. It is clear, therefore, that Dryhurst relates to a liquid based centrifugal separator similar in principle to the other references mentioned above.
There is therefore a need for an apparatus and method for separation of dry components which is simple and cost effective, avoids generation of dust, allows separation of small particles and provides for separation of components according to their morphology.